Re: [PATCH] drm/lima: Use delayed timer as default in devfreq profile

[Lukasz Luba]

On 2/4/21 1:39 PM, Robin Murphy wrote:
> On 2021-02-03 02:01, Qiang Yu wrote:
> > On Tue, Feb 2, 2021 at 10:02 PM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
> > > On 2/2/21 1:01 AM, Qiang Yu wrote:
> > > > Hi Lukasz,
> > > >
> > > > Thanks for the explanation. So the deferred timer option makes a 
> > > > mistake that
> > > > when GPU goes from idle to busy for only one poll periodic, in this
> > > > case 50ms, right?
> > >
> > > Not exactly. Driver sets the polling interval to 50ms (in this case)
> > > because it needs ~3-frame average load (in 60fps). I have discovered the
> > > issue quite recently that on systems with 2 CPUs or more, the devfreq
> > > core is not monitoring the devices even for seconds. Therefore, we might
> > > end up with quite big amount of work that GPU is doing, but we don't
> > > know about it. Devfreq core didn't check <- timer didn't fired. Then
> > > suddenly that CPU, which had the deferred timer registered last time,
> > > is waking up and timer triggers to check our device. We get the stats,
> > > but they might be showing load from 1sec not 50ms. We feed them into
> > > governor. Governor sees the new load, but was tested and configured for
> > > 50ms, so it might try to rise the frequency to max. The GPU work might
> > > be already lower and there is no need for such freq. Then the CPU goes
> > > idle again, so no devfreq core check for next e.g. 1sec, but the
> > > frequency stays at max OPP and we burn power.
> > >
> > > So, it's completely unreliable. We might stuck at min frequency and
> > > suffer the frame drops, or sometimes stuck to max freq and burn more
> > > power when there is no such need.
> > >
> > > Similar for thermal governor, which is confused by this old stats and
> > > long period stats, longer than 50ms.
> > >
> > > Stats from last e.g. ~1sec tells you nothing about real recent GPU
> > > workload.
> > Oh, right, I missed this case.
> >
> > > > But delayed timer will wakeup CPU every 50ms even when system is 
> > > > idle, will this
> > > > cause more power consumption for the case like phone suspend?
> > >
> > > No, in case of phone suspend it won't increase the power consumption.
> > > The device won't be woken up, it will stay in suspend.
> > I mean the CPU is waked up frequently by timer when phone suspend,
> > not the whole device (like the display).
> >
> > Seems it's better to have deferred timer when device is suspended for
> > power saving,
> > and delayed timer when device in working state. User knows this and
> > can use sysfs
> > to change it.
>
> Doesn't devfreq_suspend_device() already cancel any timer work either 
> way in that case?

Correct, the governor should pause the monitoring mechanism (and timer).

Regards,
Lukasz